Oh, I forgot to mention that I'd prefer the pure Thief
class, I always dislike multiclassing for some reasons (esthetical or role-playing, I don't clearly know:) ). And a Human, if possible :) I never could imagine myself as an Elf or all the more so as Orc or Dwarf :D
Hate to break it to you, but humans are probably the worst race in the game. Half-orcs have that 19 Strength thing going for them, and every other race gets bonuses to certain thief skills.
The only advantages humans have are:
1. Access to the Paladin class. This does you no good, since you want to play a Thief, not a Paladin (not to mention there's no way to be both, as the engine doesn't support it, and even if it did, the classes have incompatible alignment restrictions anyway).
2. The ability to dual class. This does you no good if you want to play a pure Thief. (You could, of course, dual class from Fighter at level 2, get weapon specialization and more HP, but still advance quickly as a thief (only being something like 4,000 XP behind). (With that said, this would cost you one level by BG1 end (which does matter somewhat, though you don't miss any critical multiplier) and one level by ToB end (which really doesn't matter at that point).
So, in other words, human is probably the worst choice for a single class Thief.
The tabletop games tried to balance this out by limiting the levels of non-humans, but that was widely regarded as a poor mechanic, and the developers of the BG series decided not to implement this particular rule (can you imaging the impact on ToB if they did?), and in any case, Thieves could still advance without limit regardless of race, so it wouldn't make a difference.
If you want a game where humans aren't the worst race (but not the only viable race due to level caps), maybe you should look for a game based off third adition (either 3e or 3.5e); there they actually balanced the races properly by giving humans some nice advantages (more skill points and a bonus feat), which is a much better approach.
If your thief will be on the frontline in BG2, then make sure to have at least a 10 or so in Intelligence as a buffer against instant death from Mind Flayer hits.
Actually, since Mind Flayers drain 5 Int at a time, 10 Int is no better than 6 Int; optimally you want an Intelligencs score that is congruent to 1 modulo 5..
With that said, there is a single use item in BG1 that boosts Int permanently by 1, and Mind Flayers don't appear until BG2, so 10 Int could work if you intend to look for that item and use it on your main character. (Such items exist for the other stats, including Strength (see my note above about 19 Str for why that's good).